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I am a PhD student at the Max Planck Institute High-Field MR Center and Faculty of Mathematics and Physics, Tübingen University.
My research interest lies within the physical aspects of functional magnetic resonance imaging (fMRI). I mainly focused on "Arterial Spin Labeling" (ASL) techniques which provide quantitative information about local tissue blood flow by tracking the inflow of magnetically labeled arterial blood into an imaging slice. Because ASL provides a direct measurement of perfusion, these methods are likely to find wider applications in studies of human brain activation mapping, disease progression, in the evaluation of pharmacological treatments.
Retinotopy describes the spatial organization of neuronal activity by providing one-to-one correspondence between each visual field location and its cortical representation. Many studies have used BOLD fMRI to non-invasively construct retinotopic maps in humans . At 1.5T and 3T most of the BOLD signal originates from veins, which might lead to a spatial displacement from the actual site of neuronal activation. In contrast to the BOLD signal, cerebral blood flow (CBF) as measured by arterial spin labeling (ASL) is less or not at all affected by remote draining veins, and therefore spatially and temporally more closely linked to the underlying neural activity.
In the present study, we aimed to determine retinotopic maps in the human brain using CBF as well as BOLD in order to compare the spatial relationship and the temporal delays of each imaging modality.
Five healthy subjects participated in the experiments on a 3T Siemens MAGNETOM Trio TIM scanner using a 12-channel head coil. The eccentricity and polar visual field maps were stimulated using expanding ring and rotating wedge stimuli. ASL images were obtained with a FAIR-QUIPSSII PASL encoding scheme with EPI readout. T1-weighted images were acquired and inflated brain surfaces were reconstructed. Voxel specific hemodynamic delay times were calculated for BOLD and CBF signals.
Figure 1 shows the retinotopic phase maps for eccentricity and polar angle stimulations on the inflated and flattened brain surface. The phase maps are color scaled corresponding to the real phase values in radians and plotted for a significantly activated visual area (z>4).
Figure 1. Retinotopic maps created by using CBF signal presented on the inflated and flattened brain surface for polar and eccentricity stimuli from the right hemisphere of a representative subject.
Figure 2 shows the comparison of hemodynamic delay times (tH) of BOLD and CBF signals for distinct visual areas.
Figure 2. A. Comparison of hemodynamic delay times (tH ) [seconds] of BOLD and CBF signals for distinct visual areas and fovea vs. peripheral sites (averaged across all hemispheres). The first bars represents the dorsal areas and the second bars represents the ventral regions of the visual areas. B. Difference of the hemodynamic delay times between BOLD and CBF signals for distinct visual areas and fovea vs. peripheral sites (averaged across all hemispheres).
The delineations of early visual areas were determined using the perfusion contrast MRI and their overlaps compared with BOLD signal retinotopy. The best match of the phase maps is obtained when the two are phase-shifted relative to each other. One reason for the deviations observed between BOLD and perfusion signals is that the BOLD signal at 3T is primarily a change in venous oxygenation whereas ASL is more closely associated with the capillary bed. While area boundaries were relatively well preserved in some of the early visual areas, we found systematic differences of response latencies between CBF and the BOLD signal .
1. Sereno M (1995): Functional MRI reveals borders of multiple visual areas in humans. Science 268, 889 - 893.
2. Cavusoglu M (2011): Retinotopic maps and hemodynamic delays in human visual cortex measured using arterial spin labeling, NeuroImage, (submitted).
, , und (Februar-2013) Regional effects of magnetization dispersion on quantitative perfusion imaging for pulsed and continuous arterial spin labeling
Magnetic Resonance in Medicine 69(2) 524–530.
, , und (Februar-2012) Retinotopic maps and hemodynamic delays in the human visual cortex measured using arterial spin labeling
NeuroImage 59(4) 4044–4054.
, , und (Oktober-2009) Comparison of pulsed arterial spin labeling encoding schemes and absolute perfusion quantification
Magnetic Resonance Imaging 27(8) 1039-1045.
, und (Mai-9-2012): Magnetization dispersion effetcs on quantitative perfusion imaging for pulsed and continuous arterial spin labeling, 20th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012), Melbourne, Australia.
, , und (Juni-2010): Retinotopy of the human visual cortex with perfusion contrast
using arterial spin labeling, 16th Annual Meeting of the Organisation for Human Brain Mapping (HBM 2010), Barcelona, Spain.
, und (Mai-5-2008): Comparison of Pulsed Arterial Spin Labeling Sequences Using Different Absolute Quantification Methods, 16th Scientific Meeting and Exhibition of the International Society of Magnetic Resonance in Medicine (ISMRM 2008), Toronto, Canada.
, und (Oktober-5-2013) Abstract Talk: An 8-Channel Transceive Head Coil Array for 7T, 30th Annual Scientific Meeting ESMRMB 2013, Toulouse, France, Magnetic Resonance Materials in Physics, Biology and Medicine, 26(Supplement 1) 345-346.
, und (Mai-10-2012) Abstract Talk: Retinotopic maps and hemodynamic delays in the human visual cortex measured using arterial spin labeling, 20th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012), Melbourne, Australia(578).
(Oktober-2010) Abstract Talk: Retinotopic mapping using perfusion contrast and velocity selective arterial spin labeling, 11th Conference of Junior Neuroscientists of Tübingen (NeNa 2010), Heiligkreuztal, Germany 13.
(November-2009) Abstract Talk: Retinotopic Mapping using Arterial Spin Labeling fMRI, 10th Conference of Junior Neuroscientists of Tübingen (NeNa 2009), Ellwangen, Germany 15.